cjwagner: Make checkconfig's cluster validation verify cluster existence and liveness based on statuses published by Plank.

JobBuild
23072
07-30 01:54
pull-test-infra-bazel 1424859844497641472 1422625672542556160 1421166971302449152 1420925537819299840
pull-test-infra-integration 1424859844594110464 1422634953794916352 1422625672664190976 1421166971407306752 1420925537932546048
pull-test-infra-verify-file-perms 1424859844522807296 1422625672580304896 1421166971344392192 1420925537852854272
pull-test-infra-yamllint 1424859844564750336 1422625672609665024 1421166971373752320 1420925537890603008